Endovascular use of devices to occlude blood vessels has become widespread both geographically around the world and anatomically throughout the body. In endovascular therapy, the doctor attempts to produce blockage or occlusion of blood flow through a vessel in order to stop bleeding. The vessel may be either an artery or a vein. His goal may be to prevent the vessel from hemorrhaging, to limit bleeding during surgery, or to stop an abnormal blood flow pattern between blood vessels (i.e. fistulas). Devices can also be used to prevent growth of abnormal protrusions from blood vessels, such as aneurysms, by creating an occlusion within the aneurysm. This occlusion minimizes or eliminates the blood pulsations which cause abnormal stresses on the wall of the aneurysm.
Several endovascular devices have been created to accomplish these goals. These devices include "glue," thrombosis producing particles, balloons, and coils. Central to the success of the device is its ability to be precisely placed within the vessel and its ability to adhere to the vessel wall. Placement typically occurs through a catheter from a proximal position outside of a patient to a distal position within the patient. Each type of device has particular attributes and particular drawbacks in its efficacy and its ability to be placed.
"Glue" refers to a group of compounds that are injected into a vessel. The glue solidifies on the vessel wall. Solidification typically occurs due to exposure of the glue to electrolytes in the blood. Therefore, glue is not actually a "device" which is solid at the time of its introduction. Control of the placement of the glue is hampered due to the variability of its cure rate within the blood stream.
Thrombosis producing particles ca also be introduced into the vessel to produce blockage of that vessel. These particles can be formed of various material such as polyvinyl alcohol, silicone polymer, protein particles, glass beads, latex beads, or silk suture material. That blockage may be temporary or permanent, depending on whether and to what degree the particle is broken down in the body, resulting in recanalization of a blood vessel after occlusion. In the case of particles, blockage occurs at the point where the blood vessel diameter is smaller than the particle. Thus, if a small particle is released into a large vessel, the blood flow will carry the particle to the point where the vessel diameter diminishes to that of the particle. This is used to advantage in tumor or vascular malformation embolization, but has the disadvantage of loss of control over the point of occlusion.
A balloon can be introduced within the vessel by a catheter and then inflated within the blood vessel to produce occlusion. The balloon may be permanently attached to the catheter, or it can have a valve at the point of attachment which closes when the catheter is withdrawn, detaching the balloon in position without producing subsequent deflation With balloons permanently attached to a catheter, the blockage generally occurs at the point of placement of the tip of the catheter, such that the level of blockage is limited to the position of the tip of the catheter. That may be far into a vascular system, such as the brain, depending on the flexibility of the catheter and the skill of the operator, but the point of the occlusion is the tip of the catheter.
With detachable balloons, the method of detachment is usually traction of the balloon against the blood vessel, producing friction which causes resistance to withdrawal as the catheter is pulled out. Alternatively, balloons can also be detached by a so-called coaxial detachment system wherein detachment occurs by advancement of a larger catheter over a smaller catheter containing the balloon. The larger catheter contacts the inflated balloon preventing the withdrawal of the balloon. This permits the inner catheter to be removed from the balloon while the balloon maintains its position. However, this system is limited to larger vessels because the stiffness of both the outer and inner catheters limits their ability to advance into ever more tortuous, distal vessel portions.
Balloon occlusion devices can sometimes deflate or can even rupture the artery in which they are introduced, thus being somewhat hazardous and unpredictable. Also, balloon devices limit embolization options by producing vascular occlusion at the time of introduction. Thus, if combined embolization is desired using both particles and a more proximal occlusive device such as a balloon, the use of the balloon precludes the first use of the particles. Thus, balloons have the advantage of control over the point of occlusion but the inability to perform combined embolization while particles have the disadvantage of a lack of control over the point of occlusion.
A more recent endovascular device for small vessels, "coils," have been used for many years to present a solution to these problems in larger vessels. A coil is typically a stainless steel wire device wound such that its outer diameter matches the inner diameter of an angiographic catheter. The coil can be introduced into a catheter in a straight configuration and pushed through the catheter with a guide wire. As it exits the catheter, it can wind itself into a "coil" type configuration. The coil produces an obstacle in the blood vessel, causing blood to clot thereon. The clot blocks the blood vessel. Further development resulted in the addition of fibers of cotton or other material within the coil, increasing its propensity to cause thrombosis more quickly.
In recent years, advancements in catheter technology have allowed progressively more distal catheterizations. However, with more distal catheterizations, the stiffness of the stainless steel coil is a limitation. In response, small-diameter platinum "microcoils" were developed. These microcoils can be introduced through the catheter with a guide wire or, alternatively, be pushed by the force of an injection of water through the catheter, thus "injecting" them into the blood vessel. Some of these "coils" are actually straight, thus enabling them to follow flow in the vessel and act more like a particle. Some are curved, thus increasing the likelihood that they will not advance beyond the point of introduction. Still, all traditional coils have the disadvantage of a lack of control, insofar as they are free objects once they are introduced into the catheter. If the coils leave the catheter tip flowing in an untoward direction or if the catheter tip moves at the time of introduction, the physician has no control over this undesirable situation or ability to recall or reposition the coil. Thus, their successful placement is extremely dependent on the skills of the surgeon/radiologist placing them.
Recently, the Guglielmi Detachable Coil ("GDC coil") by Target Therapeutics has been introduced to address lack of placement control in a limited set of circumstances. The GDC coil is attached to a guide wire by way of a solder. The guide wire is threaded through the catheter, thus allowing the operator to assure placement in a desirable location prior to detachment. When detachment is desired, a low-voltage electric current is applied to the wire, resulting in electrolytic dissolution of the solder and detachment of the coil. This is the first commercial detachable coil and is currently undergoing FDA trials.
However, the GDC coil's detachment mechanism presents several disadvantages. First, it requires the electrolytic dissolution of solder. Long-term effects of this process are unknown. Second, the process of electrolysis is time-consuming, yet treatment of aneurysms or other diseases can require placement of multiple coils. Thus, while supposedly requiring only a few minutes, as more coils are placed, interactions between devices can take place which dissipate the electrolytic current and prolong detachment. Third, the diameter of the GDC coil is 0.010". This allows a shorter detachment time but limits the applicability of the device in many parts of the body and increases the number of coils needed to achieve results. If larger coils are used, more solder would be required and presumably longer detachments would ensue. The use of 0.010" coils means the use of 0.010" catheters instead of the more widely-used 0.018" catheters in the brain and 0.038" inner diameter catheters in other parts of the body. This size restriction constrains particle sizes for introduction of other particles and limits applications to smaller vessels and pathologies.
Therefore, a need exists for a more widely applicable detachment method. This detachment method should allow utilization of larger catheters and potentially allow utilization o stainless steel coils in many parts of the body under more controlled circumstances than have traditionally been employed. The detachment method should also provide the maximum placement control as well as the ability to withdraw the device prior to detachment. The detachment method should also allow the physician to observe and verify the location of the detached device. A need also exists for an apparatus incorporating such a detachment method. The development of a reliable device for intravascular detachment would not necessarily be limited in its applicability to coils, though that would be the most immediate application.